1. Field of the Invention
The present invention relates to improved generating method and apparatus for a three-dimensional shape model. Specifically, the invention relates to an improved evaluating method and a generating method for a three-dimensional shape model which are capable of easily generating a three-dimensional shape model. More concretely, the invention relates to method and apparatus for evaluating a three-dimensional shape model using a plurality of photographed images of an object from different visual points, generating method and apparatus for a three-dimensional shape model and a computer program.
2. Description of the Related Art
In recent years, a non-contact type three-dimensional shape input device is frequently used in fields of CG and industrial design. The three-dimensional shape input device emits a laser or a pattern light to an object and captures a light reflected from a surface of the object with a sensor to closely sample and input a three-dimensional shape of the object. A real shape of the object can be put into a virtual space by the three-dimensional shape input device and a desired three-dimensional shape model can be generated easily.
The above-mentioned three-dimensional shape input device utilizes light irregular reflection characteristics on the surface of the object. For this reason, shapes of an object having low surface reflectance such as black hair of a head, an object whose surface causes a mirror reflection such as metal, a transparent or semi-transparent object such as glass and the like cannot be input correctly or their data are occasionally omitted. Moreover, since a projector of a laser or a pattern light is required, the device is expensive.
On the contrary, there is a method of photographing an object using a plurality of cameras simultaneously or photographing the object using one camera successively to generate a three-dimensional shape model using the images photographed from plural visual points without projection. According to this method, it is necessary to automatically obtain corresponding points between the images according to similarity of textures in the images. Accuracy of the detection of the corresponding points is not so good because the accuracy is greatly influenced by texture, shape, lighting condition on the surface of the object. For this reason, a user occasionally specifies corresponding points in order to heighten the accuracy. Even with this method, it is difficult to correctly obtain corresponding points of an object having a texture such that a smooth shape changes gently or a repeated pattern texture. Moreover, the object having low surface reflectance, the object whose surface causes mirror reflection, the transparent or semi-transparent object have the same fault as the three-dimensional shape input device.
A method which is called as a shape from silhouette method of estimating a shape from a shielding outline of an object can be applied to an object to which corresponding points are difficult to be given. One of the silhouette methods which is used the most frequently is a volume intersection method. As shown in FIG. 11, this method obtains a common area in a three-dimensional space within silhouettes in all images. This common area is called as a visual hull. In the most general method of obtaining a visual hull, as shown in FIG. 12, a lot of cubic shape elements which are called as voxels VX are arranged in a virtual three-dimensional space. In these voxels VX, only the voxels which are projected to areas corresponding to an object in all the images are connected so that a shape of the object is reconstructed. However, this method cannot correctly reconstruct a shape of a concave section on the surface of the object which is not shown in any image as a silhouette.
The problem that the shape is not reproduced correctly arises remarkably when rendering display is executed on a display. Namely, in an incorrect shape portion, texture data which are generated by laminating input color images become indistinct on the laminated portions or a step or a distortion occurs. In other words, on a surface portion of the object having uncomplicated texture, a distortion of the laminated textures is unnoticeable, and the distortion of the shape seldom influences an image of rendering display. From this viewpoint, Snow and the others suggest a method of smoothly connecting subsets with small scattering of pixel values between corresponding pixels in images from plural visual points in lattice points in a virtual space as a new attempt. However, in this method, the accuracy of the shape depends on a width of the lattices. When a number of lattice points is increased in order to heighten the accuracy, a data amount increases and simultaneously an executing speed drops abruptly. Moreover, the shape of the silhouette obtained by rendering the restored data becomes ragged due to aliasing of the lattices.
As mentioned above, as the method of generating a three-dimensional shape model, various methods have been suggested conventionally. However, any method has the above-mentioned problem, and a three-dimensional shape model which is suitable to high-quality rendering cannot be generated simply.